Progesterone (P) increases pituitary prolactin secretion in estrogen (E) primed monkeys. It also increases prolactin production in the decidualized endometrium. The overall goal of this research program is to determine in primates how P increases prolactin secretion, both in the pituitary and in the decidua. So far, we have shown that P increases pituitary prolactin secretion by an indirect mechanism, i.e., by activating neural systems which in turn stimulate pituitary prolactin secretion. This mechanism was deduced when we demonstrated that lactotropes do not express progestin receptors (PR). In contrast, P acts directly on prolactin gene expression in the decidua which has been shown to express PR. This project proposal is directed towards a further understanding of the neural mechanism which transduces the action of P on pituitary prolactin secretion. We have recently determined that serotonin neurons express PR and that the PR in serotonin neurons is regulated by steroid hormones in a manner that is consistent with progestin-induced prolactin secretion. Moreover, activation of the serotonin neural system increases prolactin secretion in rodents and humans. Therefore, it is of immediate interest to determine whether P alters the function of serotonin neurons. PR is a transacting factor and can be expected to alter the transcription of important regulatory genes. In this short project proposal, the expression of the mRNA for tryptophan hydroxylase (TPH), the rate limiting enzyme in serotonin synthesis, will be examined in spay-control, E treated and E + P treated monkeys using in situ hybridization and a monkey specific 5' TPH probe which we have generated with RT-PCR. According to our hypothesis, the expression of TPH mRNA will increase significantly following P administration to E primed monkeys. The increase in TPH mRNA will be correlated with an increase in prolactin secretion. In addition, the contribution of serotonin to P induced prolactin secretion in vivo will be determined using monkeys maintained in a vest and tether system. If serotonin mediates P induced prolactin secretion then administration of specific serotonin antagonists should block P induced prolactin secretion. Conversely, administration of a serotonin re-uptake inhibitor is predicted to augment P induced prolactin secretion. These predictions will be rigorously tested in a dose-related manner. Completion of these experiments will provide important insight into mechanisms regulating reproductive hormone secretion, as well as information pertaining to the action of steroid hormones on a neural system known to play a pivotal role in mood and aggressive behavior.